Patent GB-1047864-A (GB)
Title: Inter communication system
IPC: H04M9/00
Filed: 1964-02-20
Abstract: 1,047,864. Intercommunication systems; transistor gated amplifiers. STANDARD TELEPHONES & CABLES Ltd. Feb. 20, 1964 [Feb. 25, 1963], No. 7131/64. Headings H3T and H4K. In an intercommunication system comprising 4-wire substations which are interconnectible via a common talking bus, the stations have access to 2-wire exchange lines. The talking bus comprises a Go pair 28 (Fig. 1) and two Return pairs 29, 30 which are respectively connected to a microphone 35 and a left and right earphone 34, 35 at each substation 32. The dual Return paths thus allow an operator to listen to two virtually simultaneous conversations without interference. The bus is connected via electronic switches 21 to four wire lines 25, 26 which in turn are connected via suitable 2-to-4 wire converters to exchange lines 22. Selection of an electronic switch is controlled by the switch logic circuits 37, the contacts 38 of which are then separately controlled either manually or by voice frequency signals. Call to exchange (Fig. 2).-Impulses from a transmitter 40 are transmitted over the simplex of the Go pair 25 (E-lead signalling) to a detector 60 in the 2-to-4 wire converter 23 where they are suitably processed and repeated (Fig. 5, not shown) as conventional make/break signals over the exchange line 22. Call from exchange.-Ringing signals on the line are detected by an amplitude detector 55 which comprises a full-wave rectifier bridge having in each arm a PNPN diode, the breakdown potentials of which lie between the maximum voice signal voltage and the minimum ringing voltage (Fig. 3, not shown). The detector output signal is fed to a first transistorized time lag circuit 56 which serves to ensure that an actual ringing signal and not a spurious signal is present and then to a second transistorized time lag circuit 57 which serves to hold energized, during minor interruptions in the ringing, a so-called M-signalling lead, viz. the simplex of the Return pair 26. The M-lead signals operate a calling-lamp 76 at an operator position. Incoming speech is transmitted to one of the earphones 34, 35 in dependence upon the position of electronic switch 37. The " unused " earphone may be connected to another exchange line or to one of the other substations in the system via the '' unused " Return pair of the common bus 27. Electronic switches.-Each of the contacts 21 (Fig. 1) comprises a so-called contact module 150 (Fig. 8) which serves to connect an inlet 28 to an outlet 25. When the switch is OFF transistor 154 is saturated and transistor 155 and silicon controlled rectifier (SCR) 156 are " off." In order to turn the switch ON, key 182 is depressed whereby a ground-pulse is applied to the gate g of the SCR which thereby switches " on " and is followed by transistor 155. Consequently transistor 154 is no longer saturated and now acts as an amplifier for small A.C. signals. The switch is turned OFF by momentarily operating key 181. Resistor 175 serves as a volume control and a fixed amount of attenuation may be introduced by operating switch 174 in the negative feedback loop comprising resistors 172, 168, 170 and 171.